HR: 09:30h
AN: S21C-07    [Abstracts]
TI: Diffuse Plate Boundary Seismicity and Triple Junction Stability
AU: * Wetzel, L R
EM: wetzellr@eckerd.edu
AF: Eckerd College, Department of Marine Science, Saint Petersburg, FL 33711 United States
AU: Frohlich, C
EM: cliff@utig.ig.utexas.edu
AF: Institute for Geophysics, University of Texas, Austin, TX 78759 United States
AB: Diffuse plate boundaries extend over broad deformation zones hundreds to thousands of kilometers wide. For example, the boundaries are diffuse between Australia and India, North America and South America, and Nubia and Somalia. Relative velocities are slow across these regions because the pole of rotation for the plate pair is located within or near the diffuse plate boundary. This geometry causes extension on one side and compression on the other side of the pole; earthquake focal mechanisms confirm this observation. Diffuse triple junctions exist where a diffuse plate boundary meets a regular narrow plate boundary. Diffuse triple junction stability can be assessed by extending the McKenzie and Morgan (Nature, 1969) velocity vector method. In this method, lines on a map represent narrow plate boundaries and a triangle of vectors indicates relative motions between plates. In contrast, a diffuse (D) plate boundary differs from a narrow ridge (R), transform fault (F), or trench (T); it cannot be represented as a single line on a map or a unique vector on a velocity vector diagram. Because ongoing deformation is distributed over a broad zone, the diffuse boundary is drawn as a broad zone on the velocity vector diagram, and a range of stability lines is necessary to represent motions that remain in this zone. This "stability zone" usually encompasses the stability lines of the remaining two plate boundaries where they intersect. This indicates that diffuse triple junctions are stable for a wide range of plate boundary orientations. For example, in a standard velocity vector diagram, the three plate boundaries meeting at the Australia-Eurasia-India triple junction are represented as trenches. The diffuse Australia-India plate boundary, however, may be drawn along an edge or through the center of the diffuse deformation zone. On a velocity vector diagram, the diffuse plate boundary is represented as a broad "stability zone" extending between the northern and southern edges of the diffuse deformation. Because the stability zone encompasses the intersecting lines representing the other two plate boundaries, this trench-trench-diffuse (TTD) triple junction is stable. The diffuse plate boundary extends west to the Central Indian ridge where it forms either a ridge-ridge-diffuse (RRD), ridge-fault-diffuse (RFD), or fault-fault-diffuse (FFD) triple junction. All combinations are stable. Similarly, the Caribbean-North America-South America TTD triple junction and all RRD, RFD, and FFD combinations at the Africa-North America-South America triple junction are stable. The RFD and FFD triple junction combinations are stable for an Antarctica-Nubia-Somalia triple junction extending between 26 and 32 degrees east along the Southwest Indian ridge. The RRD geometry is stable through most of this region, but is unstable near 26 degrees east.
DE: 3035 Midocean ridge processes
DE: 3040 Plate tectonics (8150, 8155, 8157, 8158)
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 7299 General or miscellaneous
DE: 8155 Plate motions: general (3040)
SC: Seismology [S]
MN: Fall Meeting 2005